Re: Subtleties of __attribute__((packed))

["Phil Endecott" <phil_arcwk_endecott@chezphil.org>]

Jan Blunck wrote:
> On 12/6/06, Phil Endecott <phil_arcwk_endecott@chezphil.org> wrote:
>> I used to think that this:
>>
>> struct foo {
>>    int a  __attribute__((packed));
>>    char b __attribute__((packed));
>>    ... more fields, all packed ...
>> };
>>
>> was exactly the same as this:
>>
>> struct foo {
>>    int a;
>>    char b;
>>    ... more fields ...
>> } __attribute__((packed));
>>
>> but it is not, in a subtle way.
>>
>
> The same code is generated. [...]

I don't think so.  Example:

struct test {
   int a __attribute__((packed));
   int b __attribute__((packed));
};

char c = 1;
struct test t = { .a=2, .b=3 };

$ arm-linux-gnu-gcc -O2 -S -W -Wall test1.c

	.file	"test2.c"
	.global	c
	.data
	.type	c, %object
	.size	c, 1
c:
	.byte	1
	.global	t
	.align	2               <<<<<<<<===== t is aligned
	.type	t, %object
	.size	t, 8
t:
	.word	2
	.word	3
	.ident	"GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"


Compare with:

struct test {
   int a;
   int b;
} __attribute__((packed));

char c = 1;
struct test t = { .a=2, .b=3 };

$ arm-linux-gnu-gcc -O2 -S -W -Wall test2.c

	.file	"test1.c"
	.global	c
	.data
	.type	c, %object
	.size	c, 1
c:
	.byte	1
	.global	t                    <<<<<<  "align" has gone, t is unaligned
	.type	t, %object
	.size	t, 8
t:
	.4byte	2
	.4byte	3
	.ident	"GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"



Phil.